Photosynthetic and stomatal responses of the halophyte,Plantago maritima L. to fluctuations in salinity

Abstract Measurements of photosynthesis as a function of intercellular CO₂ (A-C₁ curve) were made on single. attached leaves of Plantago maritima L. while plants were exposed to changes in salinity. Salinity was increased in steps from 50 to 500 mol m^-3 NaCl and then returned to 50 mol m^-3 NaCl at two rates, 75 mol m^-3 (NaCl) day^-1 (experiment 1) and 150 mol m^-3 (NaCl) day^-1 (experiment 2). In experiment one, the CO₂ assimilation rate declined at high CO₂ concentrations, but the initial slope of the A-C₁ curve was unaffected in young leaves after salinity was increased to 500 mol m^-3 NaCl. The insensitivity of photosynthesis to increases in CO₂ concentration above air levels was not associated with insensitivity to a reduction in oxygen concentration. In experiment two increasing the rate at which salinity was changed resulted in larger declines in photosynthesis and leaf conductance than were observed in experiment one. Both the initial slope and the CO₂ saturated region of the A-C₁ curve were substantially reduced at high salinity suggesting that mesophyll biochemical capacity had been inhibited. However, concurrent measurements of photosynthesis as oxygen evolution under 5% CO₂ indicated no effect of increased salinity on photosynthetic capacity. This suggests that the apparent non-stomatal limitations indicated by A-C₁ measurements were artifacts caused by strong, nonuniform stomatal closure.